Improved wastewater disinfection by ultrasonic pre-treatment

For example, UV applications' efficiency is limited for samples with high concentrations of suspended matter (Fig. 1) [4]. Recent studies [6] have shown that large particles (bigger than ≈50 μm in diameter) are hard to penetrate so that the required UV demand is raised drastically. Therefore, it is common practice to install sand filters (e.g. rapid sand filters) to reduce particulate matter prior to the UV step. Rapid sand filters are expensive in construction and maintenance. They are well known in potable water production, but when it comes to wastewater treatment there are many drawbacks (e.g. clogging, algae growth, backwashing).

Another attempt to bring down the size of agglomerates of particles is the application of ultrasound (US). High power ultrasound, operated at low frequencies, is an effective means for disintegration of bacterial cells: first, at low ultrasound doses bacteria flocs can be deagglomerated by mechanical shear stress. When the US dose is increased, ultrasound cavitation can destroy cell walls. This effect is lethal to the microorganisms.

We observed in other tests that long sonication at maximum density (400 W/l) of up to 60 min leads to significant reductions of fecal coliform counts (≈3 log units). This is also in accordance with the findings of Hua and Thomson [9]. For economical aspects this method can not be considered an alternative to conventional disinfection: energy input is almost factor 500 higher than for an UV treatment with the same reduction rates.

The aim of our experiments presented here is to show that ultrasound provides an appropriate means to change the physical composition of the sample so that “big particles” are transformed into smaller ones. Therefore, we tested a combination of ultrasonic pre-treatment and UV disinfection to find out whether changes in particle size distribution of wastewater samples facilitate a subsequent UV disinfection.